Monster Pulse

"Four kids run afoul of a creepy secret organization's experiments, which turn their body parts into fighting monsters. Part sentimental coming-of-age story, part monster-training shonen manga, with just a bit of sci-fi body horror."

I read this while dealing with insomnia and it kept me invested the whole way through. It's intended for teens, but there is a lot of body horror in the content. The relationship between the main kid and her monster is understandably complicated, and you get to see the villain's entire arc. It's very cool. Since it's complete and was written over years, you get to see the cartoonist's skill improve. I always get a kick out of that.